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1995 4X4 tbi suburban

jonpdubois

By jonpdubois
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jonpdubois Enthusiast

jonpdubois

Posted
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Constant ignition miss... Replaced distributor, cap and rotor, module, plugs and wires... Runs better but still has miss... Checked for spark at all plugs and getting fire... But still a wicked miss... It misses at idle, and under load at idle... The truck moves under it's own power, but the miss becomes even more... It feels like a completely dead cylinder... And i am smelling fuel in the exhaust...did a compression test on all 8 cylinders, and the range about 135psi to 150ish psi... WHAT IS GOING ON HERE!!! Only reason i have changed what i have is due to neglect from the previous owner... He tried to fix this thing and he has no mechanical knowledge...

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

That's what I was going to suggest looking at as I read your first post - could certainly be a valve issue on the cylinder with the lowest compression. Busted valve spring, or worn out valves, bad lifters (you'll hear that one though) - any number of ailments.

 

A leak-down test is the easiest way to rule that out.

jonpdubois Enthusiast

jonpdubois

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Well... The engine didn't want to run, or stay running when we got the truck... Now you can crank it up and it will run without falling on its face or stalling... Leak down test... What am I testing and how?

Badass69 Senior Enthusiast

Badass69

Posted
Posted

If you have 135 to 150 the engine is mechanically sound at sealing for compression. So unless a rocker arm broke off the stud and a valve can't open.. or the camshaft is missing lobes it should run okay. You can pull the valve covers and see if everything is moving correctly. It could be a bad coil too as I don't see that on your list. If you replaced the entire distributor that should eliminate the pickup ring in the bottom ( not the module) as they like to crack.

 

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Jsdirt Senior Enthusiast

Jsdirt

Posted
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A leak down test will tell you the condition of the valves, head gasket, and piston rings ... but yeah that's not that bad for compression - I've seen them run fine on alot less.

 

It's not the be-all, end-all test either - it won't tell you if you have bent or broken pushrods, busted rockers, or a rounded-off cam lobe, unless you rotate the engine past the compression stroke on all cylinders. Would be quicker and easier to just yank the valve covers and watch everything while cranking.

 

Here's a cheap, bottom-of-the-barrel leak down tester: http://www.harborfreight.com/cylinder-leak-down-tester-94190.html

Badass69 Senior Enthusiast

Badass69

Posted
Posted

A leak down test will tell you the condition of the valves, head gasket, and piston rings ... but yeah that's not that bad for compression - I've seen them run fine on alot less.

 

It's not the be-all, end-all test either - it won't tell you if you have bent or broken pushrods, busted rockers, or a rounded-off cam lobe, unless you rotate the engine past the compression stroke on all cylinders. Would be quicker and easier to just yank the valve covers and watch everything while cranking.

 

Here's a cheap, bottom-of-the-barrel leak down tester: http://www.harborfreight.com/cylinder-leak-down-tester-94190.html

I know what a leak down is... lol.. it's not going to help the op at all find his miss. 135 to 150 is a healthy tbi 350.... Unless of course he oil fouls plugs instantly to the point of missing... which isn't the case by the sounds of it.

 

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Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

Was talking to the OP - he asked the question, I answered. Guess I need to use the quote box more often ...

 

Compression gauges will read different across the board - what may be 135 for him could be 90 for a good gauge - who knows. The test is cheaper than throwing a coil at it, although I suspect that might be what the issue is. But, I'd rather know 100% before spending my money.

Badass69 Senior Enthusiast

Badass69

Posted
Posted

Was talking to the OP - he asked the question, I answered. Guess I need to use the quote box more often ...

 

Compression gauges will read different across the board - what may be 135 for him could be 90 for a good gauge - who knows. The test is cheaper than throwing a coil at it, although I suspect that might be what the issue is. But, I'd rather know 100% before spending my money.

90 still won't cause a miss either.... sorry. He should pull the valve covers before buying any parts... and unless plugs are oil fouled a leak down is once again a waste of time.

 

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Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

90 when the others are at 140 surely isn't going to make it run very smooth. Maybe not a dead miss, but it ain't good!

jonpdubois Enthusiast

jonpdubois

Posted
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Well, I didn't replace the coil... And the distributor did fix the hard start and poor running condition... I am considering looking under the valve covers... It has to be getting air fuel mixture because you can smell it in the exhaust... But it checked out on compression test... When you turn it over with no spark plugs in block, and tester on one cylinder, you can definitely hear the compression hit... I rolled it three hits the first time, five hits the second... And results were the same... I doubt it is the coil because i am getting spark at the plugs... I'm thinking this has to be some mechanical issue... What about a valve lifting during compression and allowing gassed to escape? This would probably be due to a weak valve spring or bad valve face... Note that this engine is well over 300k...

jonpdubois Enthusiast

jonpdubois

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Another thing is no sound to clue a problem... Factory exhaust with no leaks... Solid truck... Engine sounds great, except for this miss... No popping or back fire... No coughing... No other symptoms... Turn key and fires right up... God awful fuel fumes from exhaust...

Jsdirt Senior Enthusiast

Jsdirt

Posted
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Usually if you have any severe valve issues, you can hear it while the engine is cranking. Should be a steady sound while cranking. If it sounds like the starter seems to speed up, then drop off for a second, that could indicate a bad cylinder. There's lots of Youtube vids on relative compression tests. I've got to get to work here fixing an exhaust on a '98 Camry. Good luck.

jonpdubois Enthusiast

jonpdubois

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The compression tester used is new... I figured there was a discrepancy with the tester and put it on my 96 350 vortech... Almost same readings... My 96 is the family daily driver and runs about 200 miles a day with no issues with 324k on the odometer... Don't think the gauge is flawed...

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